Conventional cameras with focusable objectives are provided with an outer, axially fixed setting ring rotatably mounted on a lens barrel and provided with female threads engaging male threads of an inner guide ring which is held against rotation and is rigid with a lens mount supporting the shiftable lens or lenses, the latter usually constituting a front component of the objective. The two rings are centered on the objective axis and are therefore of a rather large diameter, resulting in considerable friction along their threaded contact surfaces.
In commonly owned U.S. Pat. No. 3,482,500 there has been disclosed a control mechanism for manual and automatic focusing in which a lens mount is supported by several axially extending guide rods, one of these rods having threads engaged by a nut which is integral with the shiftable lens mount. It has also been proposed to provide a shiftable lens support with an axially extending rack engaged by a rotatable worm; see German Pat. No. 1,101,801.
In commonly owned U.S. patent application Ser. No. 894,031, filed Apr. 6, 1978 by Karl-Heinz Holderbaum, now U.S. Pat. No. 4,200,377, there has been disclosed and claimed a focusing mechanism comprising a guide rod which is fastened to a shiftable lens mount and is slidably but nonrotatably held in stationary bearings inside a lens barrel, this guide rod extending parallel to the optical axis of the objective and terminating within a camera housing in a threaded rear extremity which is engaged by a captive drive nut. The nut is provided with external gear teeth in mesh with a motor-driven pinion and also with a manually rotatable toothed wheel, the latter being rigid with a scale-carrying setting ring projecting from the housing. A slipping clutch inserted between the motor and its pinion allows the lens mount to be manually displaced even if the motor is of the self-locking (e.g. stepping) type remaining stationary when not energized. That system is advantageous for its low frictional resistance and for the accurate guidance given to the movable lens mount.